1. Field of the Invention
The present invention relates to a canned or sealed motor suitable for use in a vacuum pump whose shaft is directly coupled to the shaft of a drive motor.
2. Description of the Related Art
Conventional types of canned motor used to drive vacuum pumps are shown in FIGS. 8 and 9. Typically, the canned motor has a water-cooled cast iron motor frame 3 whose connection end is attached to a casing 1 of the vacuum pump by means of fasteners, such as bolts 2. Inside the motor frame 3, there is disposed a thin-walled can 6 of a cylindrical shape, and the opening between the can 6 and the motor frame 3 at the connection end is covered by a connection end plate 7, and the opening between the can 6 and the motor frame 3 at a free end is covered by a free end plate 8, thereby forming the stator chamber 9 and the rotor chamber 10 isolated from each other.
The stator chamber 9 houses a fixed stator 11 with its windings 12, and the rotor chamber 10 houses a rotatable rotor 14 attached to an end of a pump shaft 13. There is some clearance provided between the can 6 and the rotor 14, and an O-ring 15 is disposed between the pump casing 1 and the connection end plate 7 so that the rotor chamber 10 is maintained in a vacuum similar to the interior of the vacuum pump.
The example shown in FIG. 8 has a cooling water chamber 5 defined in a cooling pipe 4 spirally embedded in the wall of the motor frame 3. On the other hand, the example shown in FIG. 9 has a cooling water chamber 24 defined in a cooling water jacket 22, made by welding an inner tube 20 to an outer tube 21 of a corrosion resistant material, such as stainless steel, mounted on the outer wall of the motor frame 3. Details are disclosed in a Japanese Utility Model Application, Laid Open Publication, H6-88170.
The motor frame 3 is intended to provide assured cooling of the motor, as well as to perform a high precision positioning of the elements, such as the stator 11, the windings 12 and the can 6 with respect to the rotor 14, by holding those elements during the assembly of the vacuum pump.
However, all of these conventional pump frames 3 are made of cast iron, and their thick-walled construction presents a serious drawback to developing a compact and light weight motor. Additionally, in the example shown in FIG. 8, a core mold is necessary for casting the frame having a jacket structure, leading to complex manufacturing steps and consequent increase in a manufacturing cost. For the example shown in FIG. 9, because of the thick-wall separation between the cooling medium and the heat source, cooling efficiency is relatively poor, making it difficult to derive optimum performance from the motor.